Fuel assembly support system for nuclear reactor



10 Sheets-Sheet 1 J. SHERMAN ETAL uwfum un." DI

Nov. 24, 1964 FUEL ASSEMBLY SUPPORT SYSTEM FOR NUCLEAR REACTOR Original Filed Aug. 14, 1959 nnuunuc MnnHHnnHMM-n Nov. 24, 1964 J. sHERMfN ETAL FUEL ASSEMBLY SUPPORT SYSTEM FOR NUCLEAR REACTOR 10 Sheets-Sheet 2 Original Filed Aug. 14, 1959 mmm IW @www 7 j rml @i mm n. ,M M

. MM ummunnn Nov. 24, 1964 J. SHERMAN r-:TAL 3,158,543

FUEL ASSEMBLY SUPPORT SYSTEM EOE NUCLEAR EEAcToE l0 Sheets-Sheet 5 Original Filed Aug. 14, 1959 /lil :|111 xlxnllllllnhnnlnl :Inni: .L

Nov. 24, 1964 .1. SHERMAN ETAL FUEL. ASSEMBLY SUPPORT SYSTEM FOR NUCLEAR REACTOR l0 Sheets-Sheet 4 Original Filed Aug. 14 1959 7 9 4, 7 4 f 7 ,d d, j, A y; D FMNHHLM... l T w /Q MM f 1 VHHHH -n.:h QL. E Vu@ i w J 7 i l, l 66 a ww w W JY "Illlllll m fa--,\ L m f Nov. 24, 1964 J. SHERMAN ETAL 3,153,543

FUEL. ASSEMBLY SUPPORT SYSTEM FOR NUCLEAR REACTOR Original Filed Aug. 14, 1959 10 Shee'os--SheetI 5 Nov. 24, J. SHERMAN ETAL FUEL ASSEMBLY SUPPORT SYSTEM FOR NUCLEAR REACTOR Original Filed Aug. 14, 1959 lO Sheets-Sheet 6 Nov. 24, 1964 J. SHERMAN ETAI. 3,158,543

FUEL ASSEMBLY SUPPORT SYSTEM FOR NUCLEAR REAcToR Original Filed Aug. 14, 1959 10 Sheets-Sheet 7 Nov. 24, 1964 .1. SHERMAN ETAL 3,158,543

FUEL ASSEMBLY SUPPORT SYSTEM EUR NUCLEAR REAcToE l0 Sheets-Sheet 8 Original Filed Aug. 14, 1959 Nov. 24, 1964 J. SHERMAN ETAL 3,158,543

FUEL ASSEMBLY sURRoRT SYSTEM FOR NUCLEAR REAcToR Original Filed Aug. 14, 1959 lO Sheets-Sheet 9 FLE-E5 IIII Nov. 24, 1964 J. SHERMAN ETAL 3,153,543

FUEL ASSEMBLY SUPPORT SYSTEM FOR NUCLEAR REAcToR l0 Sheets-Sheet lO Original Filed Aug. 14, 1959 El Sed United States Patent O rThis application is a division of Sherman et al. application Serial No. 833,898, led August 14, 1959, now Patent No. 3,060,111, issued October 23, 1962.

This invention relates generally to nuclear reactors, and more specically to nuclear reactors designed to be used as a source or" heat in central station power plants.

It is known that -nuclear reactors can be used for the production of useful power and the 1958 Geneva Conference volume on the Shippingport Pressurized Water Reactor describes a pressurized Water reactor which has been used in a central station power plant. It is accepted at present that ysuch a power plant is not competitive economically with conventional coal and oil tired boilers. The major objective of reactor designers and engineers at present is to bring costs down to the point where economical power can be produced.

The main purpose of the Shippingport plant has always been to advance the technology of pressurized water reactors rather than to generate economical electrical power.

The particular reactor core described and claimed herein is similar to the Shippingport reactor both in construction and purpose and in fact is designed to occupy the same pressure vessel when the first core can no longer be ued. The new core embodies many improvements on the old core all of which taken together bring closer the `day that economical electric power may be obtained from the atom. Since the function of the reactor is also to obtain data to contribute to the advancement of reactor technology, provision 4is made for extensive instrumentation.

lt is accordingly an object of the present invention to develop a pressurized water reactor capable of producing power at a lower cost than those now in operation.

It is a further object of the present invention to develop a novel fuel assembly hold-down mechanism for such a reactor.

lt is also an object of the present invention to develop a reactor including means for varying the ilow of coolant through the fuel assemblies without removing them from the reactor.

It is a further object of the present invention to develop a nuclear reactor having extensive instrumentation to provide data necessary in evaluating the technical aspects of electrical power production from atomic fuel.

The reactor will first be described in general language and will then be described specifically.

The reactor consists of a pressure vessel and closure head therefor containing a seed and blanket type core containing highly enriched uranium-235 as the seed material and natural uranium in the form of uranium dioxide as the blanket material. The seed fuel assemblies are disposed in an approximately circular arrangement with blanket fuel assemblies disposed Within and around the annular seed. The shells of the fuel assemblies are identical in form. The seed asemblies include a cluster of highly enriched uranium-zircalloy alloy fuel plates while the blanket assemblies include a cluster of natural 3,l58,543 Patented Nov. 24 1964 ice uranium (U02) fuel plates. A flow of light water under pressure passes upwardly through the fuel assemblies to act as coolant and moderator. The blanket assemblies include a variable orilice device on the coolant intake side of the fuel plates so that the iiow of coolant may be varied remotely without removing the assembly from the reactor.

The reactivity of the core is controlled by hafnium cruciform control rods and fixed burnable poison located in the seed. The control rods operate in crucifonn channels provided in the seed fuel assemblies.

The fuel assemblies rest freely in a bottom support `and are held down by control rod shrouds which are provided to shield the control rods from cross flow in the upper part of the pressure vessel.

The core is extensively instrumented with some of the instrumentation leads being conducted out of the pressure vessel through the sbrouds and others being conducted through the pressure vessel through a ange held between the pressure vessel and the closure head.

The invention will next be described 'with reference to the accompanying drawings wherein:

FIG. 1 is a vertical sectional View of a nuclear reactor constructed in accordance with this invention;

FIG. 2 is a horizontal sectional View taken on the three levels indicated by the line 2-2 of FIG. 1, with instrumentation being omitted for clarity;

FIG. 3 is a fragmentary View showing that portion of FlG. 1 enclosed by the dot and dash line 3 of FIG. l;

FIG. 4 is an elevational view of a fuel assembly for the reactor;

FiG. 5 is a perspective View, partly broken away, of the top extension bracket of the fuel assembly shown in FlG. 4;

FiG. 6 is a perspective view, partly broken away, of the bottom extension bracket of the fuel assembly shown in FlG. 4;

FG. 7 is a perspective View, partly broken away, of a fuel cluster employed in a seed assembly including an instrumented fuel plate;

HG. 8 -is a sectional View showing one suba-ssembly completely and other subassemblies partially of the seed assembly shown in FlG. 7;

FIG. 9 is an elevational View, partially in section, of a typical seed fuel plate;

FIG. 10 is an elevational View, partially in section, of a seed fuel plate containing burnable poison;

FlG. 11 is a vertical sectional View of an instrumented seed plate of which the top portion is shown in FIG. 7;

FIG. 12 is a horizontal sectional view taken along the line 12--12 of FIG. l1;

FIG. 13 is a vertical sectional View of a blanket fuel assembly; e

FIG. 14 is a View partially in plan and partially in section taken along the line 14-14 in FlG. 13;

FlG. 15 is a partial horizontal sectional view taken along the line 11S-15 in FIG. 13

FlG. 16 is a View partly in elevation and partly in plan of a blanket fuel plate; y

FIG. 17 is a horizontal sectional View taken along the line 1.7-1.7 in FIG. 16;

FIG. 18 is a plan view, partially in section, taken along the line 18-8 of EEG. 19, showing the variable orifice device;

FG. 19 is a vertical sectional view of the variable orifice device taken along the `line 118-19 of FIG. 18;

HG. 20 is an elevational View 'of a control rod assembly;

FIG. 21 is a vertical sectional View of the upper portion of the control assembly;

FIG. 22 is a horizontal sectional View taken along the line 22-22 of FIG. 21;

after a loss-of-coolant accident.

FIG. 23 is a Vertical sectional View of a central portion of the control assembly showing a disconnect device;

FIG. 24 is `a vertical view, partly in cross section, of the lower portion of the control assembly, illustrating the manner in which a control rod shroud holds down the fuel assemblies;

FIG. 25 is a bottom View, partly in section, taken on the line 25-25 of FIG. 24;

FIG. 26 fis a perspective view of a corner module frame;

FIG. 27 is a perspective view of a center module frame;

FIG.'28 is a schematic plan view illustrating the module frame fuel assembly support pattern;

FIG. 29 is a view taken partly in cross section showing the bottom support;

FIG; V3O'is'aV perspective view; partly broken' away, Vof

an instrumented seed top extension bracket; and

FIG. 31 is a schematic horizontal sectional View of the reactor showing the blanket regions.

As shown in FIGS. l, 2 and 3, the reactor of the present invention comprises a pressure vessel 50, including a vessel shell 51 and a closure head 52, a core 53 contained in the vessel 50, an upper plenum chamber 54 in the top of the vessel, and a lower plenum chamber 55 in the bottom of the vessel. The pressure vessel is 32 feet in height 'and 9 feet in diameter. The core 53 contains fuel assemblies of two kindsseed fuel assemblies 57, disposed generally in a ring, and identified by the cruciform channel therein, and blanket fuel assemblies 58 disposed Within and around the ring :of seed assemblies (see FIG. 2).

The core 53 is supported in pressure vessel 50 by core support structure 59. Core support structure 59 comprises a core cage assembly 60, an upper support barrel 61, and a core support flange 62.

Core support flange 62 is clamped between shell 51 and head 52 -of pressure vessel 56 by bolts 63 extending through anges 64 and 65 on the shell 51 and head 52, respectively. Core support flange 62 is provided with wear pads 66 on the top and bottom thereof and with keys mating with keyways (not shown) in the closure head land vessel shell. This flange 62 is 19 inches in thickness.

Core support llange 62 is Welded to the top of upper support barrel 61. The upper section of upper support barrel 61 contains a'large number of vertical coolant outlet slots 67, which are distributed circumferentially about the barrel 61 in spaced relation to one another and allow thesupport barrel 61 and flange 62 to expand differentially without generating high thermal stresses. A ring of circumferentially spaced holes 63 is provided below the slots 67 to increase the exit flow area. Two rings of circumferentially spaced holes 69 are provided in the lower .portion of upper suppport barrel 61 to satisfy natural circulation requirements after loss-o-f-ow accidents and also to satisfy emergency coolant injection requirements These holes are sized and located such that'they direct streams of water at each seed assembly 57.

Upper support barrel 61 and core cage barrel 70, forming part of core cage assembly 6d, are fastened together by vbolts 71 extending through external flanges 72 and 72a formed, respectively, on the bottom of the 'upper support barrel 61 and the top of the core cage barrel 71. In addition toy core cage barrel 70 the core .cage assembly 60 includes an inner thermal shield 73,

a lateral -support ring 74, and a bottom supp-ort 75.

The core-cage barrel 70 contains and supports the bottom support 75, the inner thermal shield 73, and the lateral support ring 74. The barrel 70 is a cylinder having two axially spaced rings of oircumferentially spaced yrows of holes 76 and 77 near each of the top and bottom of the cylinder, which holes allow coolant to flow into and out of the annulus between the core cage barrel 70 and the inner thermal shield 73. The bottom support 75 is'secured to the lower end of the barrel 7u by pins 77a.

The inner thermal shield 73 is a cylinder located inside the core cage barrel 70. lt is secured at the bottom by pins 78 to the bottom support 75 and is free at the top to expand relative to the core cage barrel 70. An external iiange 79 is provided at the top of the inner thermal shield 73 to maintain the annular space between the shield and the core cage barrel 7@ and carries in a groove 80 a sealing ring 81 which prevents excessive leakage of coolant between the core cage 70 and the inner thermal shield 73.

The lateral support ring 74 is machined to iit inside the core cage barrel 70 and is attached thereto by pinsV 82. The internal contour of the lateral support ring 74 is in the form of the periphery of the core 53. The primary function of the ring 74 is to provide lateral support"'for'the fuel assembiies 5'7"'andY 58.

The bottom support 75 see FIGS. 3 and 29) bears the weight of the fuel .assemblies 57 and 58. It consists of an upper plate 83, a lower plate Sd, a ring 8S welded to the outer edges of plates $3 and S4, stiffening ribs 86 extending between and welded to the plates 83 and 84 just within the ring 85, and a plurality of support tubes 87 individually shrunk into openings in upper and lower plates 33 and 84 yand pinned to the lower plate 84 which serve as webs to make the bottom support 75 a composite structure. Support tubes 87 are arranged in the pattern of the fuel assemblies 57 and 53 which are to be supported thereby. The outer ring 85 is the component of the bottom support 75 to which the inner thermal shield 73 and core cage barrel 7i) are secured. The stitlening ribs 86 lare peripherally spaced from one another and extend radially inwardly from regions just within the ring S5 and have various widths or radial dimensions.

The support tubes 87 are shown in more detail in FIG. 29. They extend a considerable distance above the upper plate S3 of bottom support 75, and are slightly larger in diameter above this plate than below it. The inner diameter of the upper portion of the support tubes S7 is machined to accept the bottom of the fuel assemblies 57 and 5S. A tapered lead-in 91 and keyways 92 are included to faciltate insertion and positioning of the fuel assemblies 57 and 58.

In addition to serving as a support for the fuel assemblies and as webs in the bottom support 75, the support tubes 87 contain a ow meter 33 consisting of a modified venturi 94. Taps leading to the throat of the venturi 94 and taps 96 leading to the upstream side of the throat `are drilled perpendicular to the sides of the support tubes 87. Mechanical fittings 97 are attached to each tap 95 and 96 and permanently iixed to the support tube.

Above the iiow meter 93 is located a pick-up tap 93 for a sample of the coolant. Pick-up tap 9S consists of a pick-up tube 99 supported at the center of support tube S7 by a spider 100. A hole 101 is drilled through the web of the spider 100, and out the side of the support tube 87 to a mechanical fitting 102. The extension of the support tube 87 above the upper plate 83 of the bottom support 75 is to provide a calming section in the tube on the upstream side of the ow meter 93.

Holes 1&3 in the lower plate 84 of bottom support 75 provide an inlet for coolant into the bottom support. The support -tubes 87 which are located about the periphery of the bottom support 75 have openings 104 therein communicating with the interior of the bottom support to provide an outlet for the coolant from the bottom support.

A cylindrical thermal shield of stainless steel 1&5 for the core 53 surrounds the core cage barrel 7i) and is provided with an upper external flange 166 which rests upon a ledge 107cm the inside of the pressure vessel shell 51. Holes 1&3 near the top of the shield 165' provide flow channels through which coolant may pass..

Bolling flange '72a of core cage barrel 70 extends over flange 1116 of shield 105. To prevent leakage of coolant at the upper ends of the barrel 70 and the shield 105 from the annular space between the shield and the barrel 719, a Belleville spring 1119 is installed so that it is clamped by and between the thermal shield flange 1116 and the core cage barrel flange 72a.

A flow baffle 110, consisting of a foraminous plate 111, swirl vanes 112, and flow deflectors 113, is formed as an integral bottom of the thermal shield 165. Swirl vanes 112 are slightly cup-shaped and tilted so that they direct water to the bottom of the pressure vessel 59. Flow deflectors 113 are triangular in shape and direct approximately of the coolant flow past the thermal shield 165.

The pressure vessel Sti is provided with four inlet nozzles 114 uniformly spaced thereabout and communieating with the lower plenum chamber and four outlet nozzles 115 uniformly spaced thereabout and communieating with the upper plenum chamber 54. The coolant flow is supplied by four pumps which circulate the coolant through the reactor vessel and through four separate loops, each of which contains a pump and a heat exchanger. The secondary coolant obtained from the heat exchangers is used to generate electricity as in conventional in the art. The arrangement of four separate loops each including a separate inlet nozzle and a separate outlet nozzle provides an even distribution of flow to the core and promotes mixing of coolant prior to entry to the core.

The closure head 52 has a dome section 116 having a flattened top portion 117 and a refueling port 11S, which is closed by a head 119. The dome section 116 of the closure head 52 is a forging shaped essentially in the form of a truncated cone on the outside and a hemiv sphere on the inside. Twenty tubular housings 121B are secured in openings in the flat portion 117 of dome section 116 of the closure head 52 around the centrally located refueling port 118. The section of the dome containing the openings for the housings 120 is of suilicient thickness to provide the required hole reinforcement Without the necessity of providing bosses.

The head 11u, which closes the refueling port 113, is flat and circular, rests on a circular ledge 121 inside the port 118, and is held down by means of bolts 122.

FUEL ASSEMBLIES The arrangement of the fuel assemblies 57 and 5S is shown in PEG. 2. Twenty seed fuel assemblies 57, which can be recognized by the crosses designating the control rods and channels therefor, are arranged corner to corner in a single annulus shaped like a square with the corners pushed inward. Control rods are only necessary in the seed, since it is only in these assemblies that the uranium-235 loading is high enough to exceed the ssion rate required for criticality. There are 77 blanket fuel assemblies 58 occupying the space inside of and surrounding the seed annulus.

The fuel assemblies are. positioned and supported within the core cage assembly dil. They engage the support tubes 87 of the bottom support 75 and are held down by the vtwenty control rod shrouds and a center support as will be described hereinafter.

A complete fuel assembly 57 or 53 is shown in FIG. 4. It contains a top extension bracket 123, a fuel cluster 124 or 124:1, and a bottom extension bracket 125. The basic structure of all fuel assemblies is identical regardless of whether the fuel cluster 124 or 12d-a contains the natural uranium of a blanket fuel assembly 58 or the enriched uranium of a seed fuel assembly 57. Therefore the parts of each seed assembly 57 are completely interchangeable with those of each blanket assembly 58. ln addition, the general design of the three components permits either top or bottom extension brackets to be fastened to either end of the fuel cluster. The

ti ability to interchange fuel assemblies permits maximum utilization of fuel.

The fuel cluster 124 or 124:1 does little more than contain and support the fuel elements and direct coolant flow past them. They consist of plate type fuel elements arranged in a single welded unit. Spaces between plates are passages for coolant flow. The clusters are 73/s inches square with a 1A inch gap between adjoining clusters.

The extension brackets 123 and 125 provide the mechanical requirements of the fuel assembly. The functions of the extension brackets include supporting the fuel assembly in the core 53, providing means for instruinenting, directing and regulating the coolant flow and providing means for handling the fuel assembly.

The top extension bracket 123 is shown in FIG. 5. It is essentially a thin-walled box 126 of the same size and shape as the outer walls of the fuel cluster 12d or 124g. Located in the bottom inside corners of the brackets are four bosses 127 through which the top extension bracket 123 is bolted to the fuel cluster. Extensions 12S of the bottom surfaces of the top bracket 123 at the bosses 127 project into the fuel cluster 124 or 124e to key and align the top extension bracket 123 to the fuel cluster. The bosses contain a bolt clearance hole and counterbore 129. In the region immediately above the bosses 127, the corners of the box 12o are removed as at 131) to permit installation of the bolts from the outside of the bracket. Cover plates 131 inside the box section 126 isolate the bolts from the assembly coolant flow. At the top of the bracket is a relatively thin-walled section 132 with a thicker section 133 located therebelow creating a ledge 13dupon which the restraining force from the control rod shrouds and center support is applied. The thinwalled section 1532 contains four olf-center slots 135 by which the fuel assembly may be handled by an extraction tool. On the outside corners of the thick-walled section 133 are bearing pads 135 which contact similar pads 135 on ythe fuel assemblies and thus limit the lateral motion of the fuel assemblies. Also present are four olf-center apertures 137 in the thick-walled portion 133 of the box 125.

So far, the top extension brackets 123 for the seed fuel assemblies S7 and the blanket fuel assemblies 58 are identical. The top extension brackets 123 of the seed assemblies contain four inward projecting pins 13S in apertures 137. They are displaced from the center-lines so as not to interfere with control rod operation. The purpose of the pins 138 is to key the control rod shroud to the fuel assembly, thus aligning the control rod channel in the fuel assembly with the control rod channel in the shroud.

Six seed assemblies 57 contain fuel metal temperature tllermocouples and thereforerequire additional modifications to the top extension bracket. These modications are shown in FIG. 30. The top extension bracket is provided with seven thermocouple guide tubes 139, held in place by an upper support 141i and a lower support 141. The upper support holds the guide tubes 139 in the same pattern as the supports and guide tubes which are mounted in the control rod shrouds. The holes in the lower support 141 are positioned over the holes in the fuel plate in which the temperature is to be measured. The guide tubes 13@ provide a gradual transition between the angular orientation of the upper support 141D and the in-line pattern over the fuel plate.

For the blanket assembly, a coolant sampling rake is added to the basic top extension bracket. This is shown in FIG. 13 and will be described in connection with that figure.

The bottom extension bracket as shown in FIG. 6 consists or a flow transition piece 142, a fuel assembly support cylinder 143, a fuel assembly support spring 144, a

spring pad 145, a spring container 146, and'an orifice support ring 147.

The iiowtransition piece 142 is a thick-walled member whose function is to provide a transition in ow crosssectional area'from the round area of the support cylinder ,to the square area of the fuel cluster, thus assuring uniform flow distribution to all coolant channels.

The length over which the transition occurs is as long as practicable in order to improve the flow distribution and reduce loss in pressure head. In the corners of the top of the transition piece 142 are bolt holes 148 through which the bottom extension bracket is bolted to the fuel cluster. Keying extensions 149 protrude from the corners of the transition piece 142 and key and align the bottom extension bracket 125 to the fuel cluster 124 or 124a as the extensions 123` did for the top extension bracket 123.

From the bottom of the transition piece 142 extends the circular fuel assembly support cylinder 143. Two Stellite wear rings 150 and 151 are attached to the outside of the cylinder. Wear ring 150 is located at the bottom of the cylinder and is chamfered to facilitate insertion of the cylinder into the support tube S7 (FIG. 29).

The 'second wear ring 151 is located just below the spring container 146. Fastened inside the fuel assembly support cylinder 143 below the transition region is the orifice ysupport ring 147 that is used for the blanket oriicing.

This ring will be explained hereinafter. Surrounding the cylinder 143 and bearing against the lower surface of the transition piece 142 is the fuel assembly support spring 144. Enclosing the support spring 144 and a portion of the transition piece 142 is the spring container 146. lConnected to the bottom of the spring container is the spring pad 145, whose inner diameter contains two Stellite piston rings 152 which bear on the outside surface of the support cylinder to seal against leakage ow. When the reactor is assembled, the weight and the restraining load of the fuel assembly is transmitted to the bottom support through the fuel assembly support spring and pad which takes up manufacturing tolerances and relative motion due to thermal expansion. This pad, resting on the top surface of the support tube, provides a seal against leakage at this location.

Modifications to the bottom extension bracket required when it is used with a blanket fuel assembly will be described in connection with FIG. 13.

A seed fuel cluster 124 is shown in FIG. 7. The particular cluster shown includes an instrumented fuel plate 153, shown in FIGS. 11 and 12. It is two ordinary fuel plates thick and contains two fuel portions 153a, which are formed of an alloy of zirconiumand highly enriched uranium, are 0.0135 inch thick, and are spaced 0.126 inch from one another. The instrumented plate 153 also contains seven wells 154 which lie between the fuel portions 153a and extend fromA the top of the plate to a point within the plate and slant at an angle to the vertical. All of these wells are arranged in a single vertical plane and the angle of slant and depth of each well is such that the bottom of each well is directly below the one above it. Thermocouples 155 are shown in place in the bottom of the wells 154 to obtain the centerline temperature of the selected fuel plate. From this temperature distribution, the axial thermalrneutron ux and power distribution canV be obtained. Also the surface temperature of the fuel element can be computed.

The seed fuel clusterL 124 consists of a Zircalloy-Z weldment of four alloy fuel plate subassemblies 156, welded together to form a cruciform control rod channel 157. As shown in FIG. 8, each fuel plate 158 has enlarged side edges 159 and 160. In preparing the fuel `plate subassemblies 156, these fuel plates 158, together with two Zircalloy-Z end plates 161 and 162, are welded together. The end plates 161 form parts of the outer walls of the cluster 124, and the end plates 162 are interior to the cluster 124 and form part of the con- 8 trol rod channel 157. These subassemblies 156 are welded together with spacers 163 to form the control rod channel 157. The cluster 124 has two square tubular end parts 164 and 165, which are welded to the ends of the fuel plate subassemblies 156. Within the end parts 154 and 165 the control rod channel 157 is defined by extensions 166 of the four interior end plates'162 and by extension plates 167 attached to the enlarged side edges of the fuel plates 153 interior to the cluster 124. As is evident from FIG. 7, cluster end parts 164 and 165 contain bosses 168 toiwhich the top and bottom extension brackets 123 and 125 are bolted. The bottom cluster end part carries an orice member 169 in rthe control rod channel 157. This orifice member is a Zircalloy plug with a series of holes 170 drilled therein to permit a sutiicient amount of coolant ow to enter this channel 157 to cool the control rods.

Each seed isubassernbly 156 contains 24 fuel plates. The arrangement of the subassemblies is such that the orientation of fuel plates in diagonally opposite subassemblies is the same; however, the fuel plates in adjacent subassemblies are perpendicular to each other. The reasons for orienting the subassemblies in this manner are both to insure that the fuel cluster will have equal rigidity about both of its principal axes and to permit uniform zone loading of the cluster.

The arrangement of fuel plates in a seed subassembly is shown in FIG. 8. The six fuel plates 152m nearest the controll rod channel 157 may have a reducedfuel concentration to optimize thermal performance of the reactor while the seventh plate 153i) from each end is a poison plate containing a burnable poison in a rectangular area 172 at each side of the central fuel area. The remaining fuel plates 15th: contain the normal concentration of fuel. By zone loading thel seed inV this manner la power peaking factor reduction in excess of 16% over a uniformly loaded seed may be obtained.

A typical fuel plate 15Sa or c is shown in FIG. 9. It is conventional in character, consisting of an alloy of zirconium and highly enriched uranium as the fuel portion 173 thereof and a cladding 174 of Zircalloy-2. The concentration of the uranium is determined by the desired power and seed lifetime. The only difference in plates 158m from plates 158C is that they may have a lower concentration of fuel therein. The thickness of each plate 155e is 0.059 inch; the fuel thickness is 0.027 inch; the clad thickness is 0.016 inch; and the plates are located 0.67 inch apart to provide a coolant channel therebetween.

A typical poison plate is shown in FIG. 10. It consists of a fuel portion 173, cladding 174 and two areas 172 in which a burnable poison is distributed. The burnable poison is a boron-lO-stainless steel alloy with the boron being uniformly distributed therein in the form yof 1 to 5 micron particles. A thin barrier of niobium is employed to separate the burnable poison from the fuel. In the alternative the boron may be incorporated in a burnable poison matrix as a powder dispersion of Fe2B1 particles of controlled size and distribution. As a third alternative B410C pellets or wafers may form compartments in the burnable poison matrix.

Burnable poison is located in two of the 24 fuel plates 158 of each seed subassembly 156 and the core 53 contains a total of approximately 800 grams of boron-10.

A complete blanket fuel assembly 58 and associated instrumentation is shown in FGS. 13-19. The blanket fuel assembly `58 includes a topl extension bracket 123, a fuel cluster 12451, and a bottom extension bracket 125 which are 'identical in basic form to these elements in the seed assemblies 57.

As has already been stated, the only modification required in the basic top extension bracket 123 to use it in the blanket fuel assemblies 58 is to incorporate a coolant sampling rake 175 therein. This is pinned in place by pins 175 which occupy the same holes that pins 13S occupy in the seed fuel assemblies 57. Coolant sampling rake 175 comprises a stainless steel box 177 mounted in the top extension bracket so that it covers the complete cross-sectional ow area. Its function is to gather a representative sample of coolant from the fuel assembly flow stream. Circular tubes 173 pass through the rake 175 to permit the bulk of the coolant flow to pass through the rake. The upstream side 179 of the rake 175 is perforated with many small holes 1811 (see FG. 14) through which a sample of the coolant is removed and flows into the interior of the rake. A tube 181 located in the center and perpendicular to the sides of the rake 175 extends down through the fuel assembly 58. This tube 181 communicates with the interior of the rake by means of openings 132 in a Stellite bushing 183 and openings in the tube 151 registering with openings 182. It is through tube 181 that the sample is removed from the rake. The Stellite bushing 183, which is mounted in the rake, supports the sampling tube 181 and provides a seal to limit leakage into the rake. The sampling tube 131 may be manipulated by a tool, not shown, connectible with bayonet slots 1&4 in the upper end of the sampling tube.

The sample passes through the tube 151 through the center of the fuel assembly and bottom extension bracket 125. As shown in FIG. 29, the sample is transferred from the sampling tube 131 to the pick-up tap 93 by means of a sphere-in-cone type of seal 135 made with a compression spring 186 carried Within a spring housing 185e (see FIG. 19) in the bottom extension bracket 125. The spring load is applied to the sampling tube through a ange 187' on the tube 181 in the region of the variable orifice.

Each blanket cluster 124e contains two rectangular subassemblies 153 welded together to form a square structure and two cluster extensions 189 and 1943' welded to the ends of this structure. Each fuel subassembly 155 has a groove 191 machined into one side wall and extending the full length of the subassembly. When two subassemblies 15S are joined together, the grooves 191 register with one another and form a small opening 192 through the center of the cluster 124m through which opening the coolant sampling tube 181 extends.

Each subassembly 188 contains ve central plates 193e', 18 standard plates 19315, and l0 modified plates 193C. Each of plates 193:1, 1935, and 193C has enlarged side edges 194 and 194:1, which are welded together and to end plates 195 to form the two subassernblies 18S and to join the latter to one another to form the fuel cluster 124g. As shown in FIGS. 16 and 17, plates 193 are composed of uranium dioxide fuel wafers 195 situated in compartments 197 formed by structural ribs 198 to which the cladding 199 is bonded. These blanket fuel plates embody the inexpensive, corrosion-resistant, irradiationdamage-resistant, U02 fuel and the favorable heat transfer characteristics of plates. As shown, ribs 19S are in a rectangular lattice in which the length of the compartments formed by the lattice is much greater than the width. These compartments and the wafers contained therein are of different sizes depending on their location in the blanket subassembly.

All plates contain 88 individual compartments 197 arranged in eight rows of eleven each. One row of cornpartments 2Go at one end of the fuel plates is of shorter length than the rest of the compartment rows. In assembling the subassembly 183 alternate plates 193a, b and c are inverted so that the horizontal ribs 198 of two adjoining plates are not situated at the same level. The reason for this is to minimize the restrictions which occur in a water channel due to possible protrusions of the horizontal receptacle ribs above the normal heat transfer Surface of the plate which would restrict the width of the coolant channel between the plates. This possible protrul@ sion is the result of tolerance accumulations which occur during fabrication of the plates.

As has been stated, three dierent kinds of blanket plates have been employed. The standard plate 193]) has a nominal thickness of 0.138 inch including a fuel thickness of 0.150 inch. Modiiied plates 193C have a nominal thickness of 0.124 inch including a fuel thickness of 0.686 inch. By this means a reduction of 8% in the local power peaking factor over a uniformly loaded blanket is obtained. The center plates 193e have the same thickness as the standard plates 1935 but a shorter span to provide the opening 192 at the center of the assembly. The rib thickness is 0.102 inch and the plates are 0.076 inch apart to provide a coolant channel therebetween.

Each of plates 193:1, 1931), and 193C is 91 inches in length, with plates 193b and 193C being 3.658 inches in width, while plate 193e is narrower to allow for opening 192. The normal length for a compartment is 12.229 inches while the short compartment 25h is 3.494 inches in length. The width of a compartment in plates 193e and 193e is .246 inch and in plates 19311 is .256 inch.

The fuel wafer is a refractory, nonmetallic unit of natural U02 having a minimum density of 96% of theoretical. The length of each wafer is approximately 11/2 inches; therefore, two wafers are present in the shortened compartment 25@ while seven are present in the normal length compartment 197.

As shown in FG. 31, the blanket fuel assemblies S8 are divided into four regions according to their position relative to the seed. The thermal neutron flux distribution is uneven in the blanket With the highest neutron flux occurring in the blanket regions nearest the seed and decreasing with distance from the seed. To obtain optimum power the coolant ilow through each of these four regions must differ with the highest flow passing through the assemblies exposed to the highest flux. The variable orifice devices, which are located in each blanket fuel assembly 58 as will next be described, are employed to establish the amount of iow through each of the regions of the blanket. By establishing four regions of oriiicing, of the core power which could theoretically be obtained by establishing a larger number of regions of orificing is attained.

In the lower extension bracket of the blanket assemblies 58 is located a variable orifice device 21115. Since the average power density in the Seed is larger than in the blanket, approximately 50 watts/ cc. and 17 Watts/ cc., respectively, the minimum flow required to cool a seed assembly 57 is larger than for a blanket assembly 5.5i even though the heat transfer area per seed assembly is larger than for a blanket assembly. In order to reduce the limited ow available, the blanket How is reduced accordingly. The variable orifice device makes it possible t0 vary the flow through the blanket assemblies without disassembling the reactor.

The variable orifice device 255 is mounted on sampling tube 181 and operated thereby. It consists of an odd number, for example, three, of spaced annular plates 256e having arcuate orifices 207 therein, the spring housing 185:1 depending from the lowermost plate 2565i, an outer cylindrical wall 208 joining the outer edges of the plates 2tl6a, an even number, for example, two,I of spaced annular plates 236k having orices 257 and alternating with the plates 255e, an inner tubular hub 2419 joining the inner edges of the plates 2565 and being mounted on the sampling tube 181, two sets of key slots 21h located 180 apart formed at an internal corner between the wall 258 and the upper orifice plate 256e, two keys 211 located apart on the periphery of the adjacent orifice plate 25617 which are engageable with the key slots 215, the spring 186 bearing on flange 187 which functions to urge the key 211 into engagement with one of the key slots 210, four through vertical grooves 212 spaced 90 from one another on the interior of hub 299, four blind vertical grooves 213 spaced 90 from one another and 45 from the through grooves 212 on the interior of hub 299, and four projections 214 spaced 96 from one another on the sampling tube 131. The orifice device 235 is supported in the bottom extension bracket 125 below the ow transition piece 142 by a support tube 215 and integral spider 216, an orice support ring 147 and a wave washer 218. The spider 216 is located near the top of the extension bracket 58 and is held in place at the joint between the bracket 58 and blanket fuel cluster 124. The support tube 215 extends from the center of the spider 216 and holds the bottom surface of the oriee unit against the compressed wave washer 218 on the orifice support ring 147 secured in the fuel assembly support cylinder, 143. Tolerance build-up between the orilice device 205 and bot-tom extension bracket 125 is taken up by wave'w'asherV 21S.V t

The orifice plates Zb are rotated relative to the fuel assembly 188 while the orilice plates Ztltla are perinanently fixed with relationship to the fuel assemblies. In order kto change the pressure drop or" the variable oriiice device 225, the two movable plates 26611 are relocated with respect to the stationary plates 26661. This changes the position of the orifices 207 in plates 2%@ with relationship to those in the plates 2Mb. The minimum pressure vdrop occurs when the oriiices 267 in the movable plates 206b are in complete registration with the orifices in the stationary plates 206e, and the pressure drop increases with decrease in registration of the orices of the movable plates with those of the stationary plates. rlhe required degree of registration of the orifices in the plates is predetermined. The orifices 2G17 are arcuate in shape so that changes in the degree of registration of the orifices give a linear variation in the pressure of the coolant flow therethrough.

In order tovary the registration of the orices 207 without removing the blanket elements 58 rfrom the reactor, the bayonet slots 184 at the top end of the sampling tube 181 are engaged by a tool. The sampling tube 1&1 is then lifted by the tool, thereby compressing the orifice spring 136 and moving the projections 211i axially beyond, and thus out of engagement with, the through slots 212. The sampling tube 181 is then rotated by means of the tool till the projections'Zl/l are aligned withv the blind slots 213. The sampling tube is then lowered while the projections 214 engage the blind slots 213. A downward force is then applied to the sampling tube 181 moving the hub 209 and the oriiice plates Ztlb which are attached thereto. This disengages the keys 211 from the key slots 210 in the stationary plate 266s Amaking it posf sible to rotate the sampling tube 181, hub 209, and mov- REACTVITY CONTROL SYSTEM Thepower level of the reactor is controlled by 20 cruciform'control rods 219 (shown in FIGS. 2O and 24) which operate in similarly shaped channels 157 provided in each of the seed fuel assemblies 57. The control rods 219 are formed of hafnium and each one` is 94 inches l long with a 5 lli -inch `span and a .22d-inch thickness. The total reactivity worth of all rods is 15.1%. Near the lower end of the rod, rubbing shoes 22@ are provided to minimize the Contact surface between the blades of the rod and the walls of the control rod channel 157. The shoes reduce the friction loads during operation and assure that coolant how is provided on all faces of the control rod should the rod be displaced to one side or distorted by uneven temperature distribution. The control rod 219 is joined to a control rod shaft assembly 221 by an adapter 222. Adapter 222 is joined to shating assembly 221 by a bayonet type connection 223. As shown in FIG. 20, the control rods 2l9 are operated by operating mechanisms 22d mounted on the pressure vesselv closure head 52 vertically'above, and in line with, the control rods. The control rods 219 are connected to the mecha.- nisms 224- by the control rod shaiting assembly 221. Since the operation mechanism 224 is'conventional it will not be further described.

As shown in FIGS. 2l, 23, and 24, control rod shatting assembly 221 consists of a tie rod 225, a control rod shaft 226 having a lower noncircular portion 227 or" small diameter and an upper circular portion 223 of greater diameter forming thereby a butler piston 229, Vand a lead screw 23). More particularly, the noncircular lower portion 227 of shaft 225 is characterized bytwo opposed parallel iat sides and two opposed arcuate sides on the same center. Tie rod 225 connects theV shatting assembly 221 to the control rod adapter 222. It passes through the lead screw 230 and the control rod shaft 225 and can be operated from outside the pressure vessel. Operation from outside the pressure vessel is required to disconnect the shafting assembly 221 from the control rod 2l@ to permit raising of the shafting assembly during refueling without removing the control rod W9 from the core. The tie rod 225 is designed to move inside the lead screw 23) and the control rod shaft 226. It can be positioned axially and rotated to perform the connecting and disconnecting operation. The tie rod 22S serves as the primary energy absorbing component of the control rod shatting assembly 221.

At the lower end of the tie rod 225 is a male portion 231 of the bayonet type connection 223 between the tie rod 225 and the control rod adapter 222. The tie rod 225y has lugs 232 thereon whose upper surfaces form a ledge 233 which mates with ledge 234 formed by recess 235 in the control rod adapter 222 which forms the female portion 23e of the connection 223. Below the ledges a pin 237 on the tie Arod 225 is guided by a J-slot 232 in the control rod adapter 222 to insure proper alignment of the mating ledges 233 and 234 during the connecting operation. Channels 234e in the ledge 234 are provided for protuberances 232 to pass through when making or breaking the connection. These are shown in FIG. 25.

The hollow control rod shaft 226 provides the connection between the lead screw 23S and the control rod 219. The upper portion 223 of the shaft 226 is 2inches in diameter and the lower portion 227 is 1% inches in diamr eter. upper portion 223 serves as the butler lpiston 229 during scram.

The lead screw 235i is a long externally threaded tube which extends along the centerline of the control rod drive mechanism 224. It has external threads 239 which are threaded into internal threads 24) in control rod shaft 226 to which it is secured by a cup washer 241;

As shown in FIGS. 1, 2G, 21, 23, and 24, each control rod 219 and shafting assembly '221 is surrounded in the upper plenum chamber 5d by a control rod shroud 242 which has a constant outside diameter of 5% inches along its entire length. rod and shafting assembly from hydraulic rorces due to coolant crossdlow and supports the shaft bearings, butler and instrumentation leads but also holds the fuel assemblies in place. This forms an important part or" the invention which will be described in detail hereinafter. As seen in FIG. 28, the assemblies 57 and 53 are held 1n place by `module frames 243m 24%, and 243C. As seen in FIGS. 24, 25, and 26, module frames 243m and Zdb are each secured to the lower end of a control rod shroud 27452 and four control rod scabbards 2da including internal flanges 244e are secured within each shroud 242.

The transition between lower portion 227 and This not only shields the controlv Module frame 243e will be described hereinafter, As shown in FIGS. 2l and 25, each control rod shroud 242 comprises upper, middle and lower tubular sections 245, 246, and 2417 and a buffer The module frame Zda or 24-3b is Welded to the lower section 247 of the shroud 242, as shown in FIGS. 24 and 26. The control rod 239 never cornes above'the top of lower section 247 of the shroud 242.

The shroud 242 contains instrumentation guide tubes 250 and the upper section 245 thereof contains the lower end of a mechanism housing 249, which is threaded into shroud support flange member 251, which in 'turn is threaded into upper section 245 of the shroud 242. and pinned therein by pin 253. Flange 252 rests on ledge 25d in housing 120.

At the lower end of mechanism housing 249 is a lead screw bushing 255 immediately surrounding the lead screw 236 while immediately thereabove is an instrumentation shield tube 2545 containing instrumentation holes 257. The function of shield tube 256 is to protect the thermocouples passing through the mechanism housing 249 from damage that could be caused by the lead screw 23?.

To guide and support instrumentation guide tubes 256i, a support plate 25d is pinned to the upper shroud section 245 by pins 259 just below mechanism housing 25x52. The control rod shroud 242 is clamped in the closure head 52 by a ring 260, which is threaded into the housing i2@ to tighten the flange member 25l against the ledge 251i in the housing 126B. Lock washer 261 prevents rotation of the ring 260 and a closure ring 262 on top of the lock washer 261 is threaded into and welded to the housing l2@ to prevent leakage of coolant therefrom.

Tapered bearing 263 forms a part of a block 264 which connects upper section 2.45 and middle section 242-6 of the shroud 242. The connection is made by threads 265 and pins 266. The block 261% includes flow apertures 267 for coolant and instrumentation apertures 265 through which instrumentation guide tubes 25d are led.

Because each of the module frames 24E-3a, 243mb, and 243C is larger than the inside diameter of the housing 129 in the closure head S2, the shroud 242 is made in the three separate sections 2li-5, 246, and 247 so that the sections 2426 and 246 may be disconnected from the section 2437 and removed through the housing lZl, and the section 24.7 and module frame 24351 or 243k, being permanently joined to one another, are removed through the central refueling port ll, which is of relatively large diameter.

For removal of the shroud tube and module frame separation is made by shroud disconnect assembly 2&9 which includes bilder block 274B containing a buffer cylinder 271 which cooperates with buffer piston 229 to serve as the control rod bufer 248.

Impact surface 272 of the control rod shaft 226 is a conical Surface with a 126 included angle. The impact surface 273 in the butter block 274i has a curved face whose chord is inclined to mate with the conical surface of 4the shaft. This design decreases the possibility of point contact between the two surfaces which would result in excessive strains at the point of impact. The buffer action takes place at the last l2 inches of total control rod travel. Therefore, 86.5% of free fall is provided prior to slowing down by the buffer.

Advantages of locating the buffer in the shroud are the following:

(l) Operating conditions have little or no effect on the buffer performance. It is not necessary to have the pressure vessel completely full and at some minimum pressure for the butler to be operative.

(2) The mechanism design is simplied.

(3) The drag on the control rod shafting is decreased during scrarn by eliminating close clearances during the full travel ofthe shafting.

Since the buffer is a hydraulic damper, it is necessary that it be submerged in water to be operative. The minimum water level is 84- inches above the outlet nozzles ll' of the pressure vessel.

In addition to the buffer block 27@ the shroud disconnect assembly 269 includes a torque restraint bearing 274 and bearing retainer 275, a shroud locking sleeve 27 f and locking sleeve retainer ring 277, and a shroud adapter 273 having functions which are obvious from their names. The bearing 27dn has the same noncircular shape as the lower portion 227 of the control rod shaft 226, so that the shaft 226 cannot rotate with respect to the assembly 26E?. Also butler blocks 27@ and lower shroud adapter 27S have coolant flow apertures 279 and instrumentation apertures 27Std therein.

To remove the shroud 242 from the pressure vessel, the shroud is lifted as high as it can be lifted, then it is disconnected at the disconnect assembly 269, the upper and middle portions 2&5 and 2426 are removed through the penetration housing l2@ and the lower portion 247 of the shroud 24E-2. and the module frame. 2436i or 2%!) are removed through the central refueling port lr.

Secured to the inside of the lower shroud tube 247 are the four scabbards 24d or How baies which form the control rod channel in the shroud. Scabbards 244 in cross section are roughly a flattened semicircle with internal flanges .i2-fida which are welded to the lower tubular section 247 of the shroud 242. These scabbards act as a shield to protect the control rod from hydraulic forces due to cross-flow in the upper plenum chamber. The shrouds have openings 27% therein in all three portions thereof to permit liow of coolant from the shroud.

FUEL ASSEMBLY SUPPORT SYSTEM In order to simplify the construction of the reactor, components already present in the reactor are used to hold the fuel assemblies S7 and 53 in place. These components are the control rod shrouds 242. In addition to the shrouds one additional support member, center support Zti, is employed. In this system of support the fuel assembly bottom extension bracket engages support tube 87 in the bottom support 75', making the fuel assemblies free standing members. Depth of engagement is 6 inches. At the top of the fuel assemblies lateral clearance is limited by the lateral support ring '74 which lls the void between the inside of the core cage barrel 7d and the outer periphery of the fuel assemblies. Hydraulic and mechanical loads acting on the fuel assemblies are transfered to the shrouds 242 and center support 28@ through the module frames 243m, 243.6, and Zdc. The module frames are all welded in construction and fabricated from stainless steel plates. Bearing pads dl are machined on the bottoms of the module frames 243 so as `to minimize the ellect of the fuel `assembly loading on the supports. Loads are transferred from the fuel assembly to the shroud by means of ledge 134 on top extension bracket L23. There are three types of module frames required to secure all fuel. assemblies in the coreside module frames 243m shown in FIGS. 24 and 25, corner module frames 243!) shown in FIG. 26 and center module frame 243e shown in FIG. 27.

As shown in FIG. 29 the core cross-section is divided into repeating patterns or groupings of fuel assemblies for the purpose of support. All but one of the groupings, the center group, contain one seed assembly 57 and either three or five blanket assemblies 5S, depending upon the location of the seed assembly in the core. A single inline support pattern is established for 16 of the 20 seed locations. This pattern includes a seed assembly 57 with two blanket assemblies 58 on one side and one blanket assembly on the opposite side, The four remaining seed locations not covered in the in-line groupings are at the locations in the core where the in-line groupings change direction. vere the seed assembly 57 is located in the Y semblies.

` corner of an angular arrangement that contains six asrlfhe module frames associated with the inline and angular groupings are called the side module frame 243e and corner module frame 24315, respectively.

The analysis of the shroud support patterns indicated that it is impractical to hold down the center part of the core with the control rod shrouds 242. The center nine blanket assemblies 58 are so far removed from the nearest seed assemblies 57 that very large shrouds would be required to restrain the unbalanced loading produced. For this reason, a support member, the cen-ter support 28d shown in FIG. 1, is extended from the central refueling port hold 119 to secure the nine central blanket assemblies. The module frame welded to this support is called the center module frame 243C.

As shown in FIGS. 24 and 25, each of the side module frames 243e comprise two parallel side plates 282 and three parallel uniformly spaced cross plates 283 at right angles to the side plates forming two square openings 284 through the frame. There are short extensions 285 t0 the side plates 232 beyond the cross plates 283. This frame 243a is welded to shroud 242 by lugs 236 so that the shroud is centered Vin one of the square openings 284 in the frame. Location of the bearing pads 2%1 so that they will bear'on one seed assembly 57 and three blanket assemblies S8 is shown in FlG. 25. Coaxial with and having the same radius as the shroud 242 is a shroud alignment ring 287 which has four shroud aligning slots 283 therein displaced from the centerline. These slots receive pins i355 in the upper bracket 123 (FIG. 5) to key the control rod shroud 242 to the seed assembly 57, thus orienting the control rod channel 157 in the seed assembly 57 with the channel formed by the scabbards 244 in the shroud 242. Shroud alignment ring 287 is secured to the scabbards 244.

As shown in FIG. 26, corner module frame 243i; cornprises two outer side plates 239 and 29) which join at right angles, two inner side plates 291 and 292 which also join at right angles and are parallel to the outer side plates 239v and 290, respectively, and four cross plates 293 of which two are parallel to plates 239 and 291 and two are parallel to yplates 22@ and 292 thereby forming three square openings 294 in the frame. Side plates 239, 290, 2% and 292 each have a short extension 295 which extends beyond the cross plates 293. A shroud alignment ring 287 having shroud aligning slots 23g is provided as in the side module frames 243:1.

As shown in FIG. 27, the center module frame 243C comprises two pairs of parallel plates 296, the plates of lone pair being at right angles to those of the other pair to form a square opening 297 therein. Each plate 296 has two short extensions 29S beyond lthe other two plates 296 that extend at right angles thereto. The module frame 243e is not connected to a shroud 242 and therefore does not include a shroud aligning ring. lt is instead welded to center support 28) by lugs 299.

As shown in FlG. l, slots 299e are provided along the length of support 2%@ to permit exit of coolant water from the center blanket assembly over which it is positioned and to prevent vibration duc tocross ow in the upper plenum of the core. The center support 280 is secured at its top to the central refueling port closure 119.

The arrangement of bottom support 75, fuel asse blies 57 and 5S and control rod shrouds 242 makes possible individual handling of damaged or spent fuel assemdisassembling the core to take advantage of the power shift from the seed fuel assemblies 57 to the blanket fuel assemblies 58 which occurs as the seed becomes depleted and plutonium is produced in the blanket.

IJSTRUMENTATION With the use of the core support tiange 62, the core 53 can be very extensively instrumented. Removal of the bulk of the instrumentation leads through the core support Flange d2 allows a simpler closure head design and permits simpler refueling procedures, because it is not required to disconnect most of the instrumentation leads when the head is removed. The use of a flange also eliminates the necessity to mount a superstructure for instrumentation on the reactor head.

Instrumentation is provided to determine the following:

(a) The ilow through and the exit temperature oi each fuel assembly S7 and 58.

(b) The inlet temperature to 2l representative fuel assemblies 57 and 5S. Good mixing of coolant prior to `entry to the core precludes the necessity of measuring provided in the seed region are provided in the struc# tural components because of the possibility of future use of a different seed. Only the blanket fuelr ass-ernblies 5S are monitored to detect and locate failed elements lin the reactor core described herein.

The core support ilange d2 is used to carry instrumentation leads for the ow measurement devices, the failed clement detection and location system, and the inlet water thermocouples. This is referred to as support flange instrumentation. The remaining instrumentation 1s re- 1Ierred to as shroud instrumentation andV includes exit water and seed metal temperatures and core differential pressure instrumentation.

The flow measuring device has already been described in connection with support tubes 87. As shown in FIG. 3, tubes Stili which connect to the fittings $7 (FIG. 29) are grouped and enclosed in a conduit Sill which physically protects the tubes during core assembly and refueling. The conduit 391 is routed over the bottom .support being held in place by supports 392. It is then routed up the inside surface of the inner thermal shield 73 and through the lateral support ring 74 and eventually to the support flange 62, as shown in FIG. l. There are eight penetrations in the flange 62 for the 194 flow measurement tubes.

Since the tubing conduit 3M is subject to the effects of heat generation, a cooling system (not shown) may be included in the conduit. It would consist of a 1/4 inch tube fixed to' the end of the conduit that extends down through both plates of the bottom support and piclrs up inlet cooling Water. Y

The failed element detection and location system (PEDAL) provides facilities to sample the coolant of each fuel assembly and pipe the sample to monitoring stations outside the reactor in order to detect and locate fuel assemblies containing failedV fuel elements. As has already been stated, a pick-up tap 93 is located in each support tube 87 and an individual piping system connects each tap with a multi-port valve located outside the reactor vessel. The piping system comprises tubing that is attached to fitting 102 of pick-up tap 9S. This tubing 1 a 9 Table I-Continued MECHANICAL CHARACTERISTICS [Used as a basis for thermal performance] Table II describes behavior of thereactor with no reoricing during the life of the core. Further power gains can be made by re-orificing at specific times during the life of the core. For one re-orificing at 5000 e.f.p.h., an increase of 8 mw. over the first 5000 hours and 4 mw. over the last 5000 hours may be obtained. A further gain is obtained by an additional re-orifcing at 8000 hours whereupon 2 mw. are gained from 5000 hours to 8000 hours and 4 mw. are gained from 8000 hours to 10,000 hours.

At various times during the life of the core it will be necessary to refuel or exchange individual fuel assemblies through the central port 118 of the pressure vessel head 52. Th'ese changes may occur when replacing a complete seed, removing individual assemblies for observation, removal of damaged assemblies, and, possibly, replacement of all assemblies. g

A dry refueling system is the preferred method for refueling. Following reactor shutdown and the coolingoff period required, the reactor pit is drained, necessary seals are cut and the head 119 removed. A transfer cask forming no part of the present invention is then installed in the pressure vessel head 52. After removal of the center support 280 from the vessel into its shielded transfer cask it is taken to storage to a fuel handling canal. Tie rods 225 are disconnected from control rods 219 in those mechanism-shroud-module frame assemblies which must be elevated during the refueling operation. The mechanism-shroud-module frame assembly is then raised to the extent of travel allowed before the module frames interfere with the bottom surface of the vessel head 52.

The -fuel assembly transfer cask and extraction tool are then positioned on the central refueling port 118. The extraction tool, `attached to :a crane, is lowered into the vessel through the transfer caslc and the selected assembly is picked up, translated to the refueling port 118 and withdrawn into the shielded transfer cask. The transfer cask is transported to the fuel storage area where the fuel assembly is lowered out of the cask. The transfer cask is returned 'and installed on the refueling port. A new fuel assembly is installed in the cask and lowered into the vessel into the vacant location in the core. Mechanismshroud-module frame assemblies are returned als soon as all assemblies in the module have been exchanged.

Following installation of the final new fuel assembly, the transfer cask is removed from the vessel. Using the module frame shielding cask, the center support is replaced in the central fuel port. The centnal port closure plate 119 is then reinstalled and seal welds made. Bolting of the central port closure plate is accomplished. The reactor pit may then be flooded and reactor startup procedures begun.

It will be noted that removal of many of the instrumentation leads through the support flange 62 makes this simple refueling procedure possible since the instrumentation leads passing through the flange 62 do not interfere with refueling in any way.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the Iappended claims.

What is claimed is:

l. A nuclear reactor comprising a core containing a plurality of vertically disposed seed and blanket .fuel assemblies, :said seed assemblies containing enriched uranium and said blanket lassemblies containing natural uranium, each of Isaid seed assemblies including a control rod channel extending axially of the assembly, control rods having operating shafts extending thereab-ove capable of axial movement in the control rod channels, tubular shrouds enclosing said control rod `operating shafts, and module frames attached to each shroud .adapted to bear on a -seed assembly and adjacent blanket assemblies to `restrain them from vertical movement.

2. A nuclear reactor `comprising a core containing a rectangular array of rectangular, vertically disposed, seed and blanket assemblies, said secd assemblies containing natural uranium, said seed assemblies being disposed generally in la ring about a central blanket region and being surrounded by a peripheral blanket region, each of said seed assemblies including a control rod channel extending axially of the assembly, control rods capable of axial movement in said control rod channels, said control rods having operating shafts extending thereabove, tubular shrouds enclosing said operating shafts, module frames attached to the bottom of each of said shrouds each adapted to bear on a seed assembly and adjacent blanket assemblies to restrain them from vertical movement, a central support extending from the top of the fuel assemblies to the top of the reactor, and a module frame attached to the bottom of said support adapted to bear on a plurality of blanket `assemblies in the central blanket region to restrain them from vertical movement.

3. A nuclear reactor laccording to claim 2 which includes 20 seed assemblies and 77 blanket assemblies and wherein the central module frame bears on nine blanket assemblies, each of four corner module frames bears on one seed assembly and five blanket assemblies, and each of 16 side module `frames bears on one seed assembly and three blanket assemblies.

References Cited in the le of this patent UNlTED STATES PATENTS 2,920,025 Anderson Ian. 5, 1960 2,992,174 Edlund et al July 11, 1961 2,999,059 Treshow Sept. 5, 1961 `out interfering with any other reactor component.

is routed through conduits similar to those shown for the ow measurement instrumentation. These conduits (not shown) are routed along the top of and at right angles to the flow measurement conduits 301. The tubes are then carrried up along the inner thermal shield73 and out of the reactor through the support flange in the same manner as the flow measurement tubing. There is one penetration in the an'ge 62 for the 97 PEDAL tubes.

The inlet Water temperature is taken at 21 fuel assembly locations. The thermocouples 303 (see FIG. 29) are located in the flow stream of the fuel assembly, mounted in the same plane as the spider 100 of the coolant sampling tap. The thermo/couples 303 are laid singly along brackets (not shown) placed on top of the FEDAL lines, then converge in conduits that extend up the side of the reactor and out' the ange 62.

Thermocouples (not shown) are also provided to Y measure the temperatures that indicate the temperature gradient and heat generation in the structural components of the core. These thermocouples measure the temperature of the core barrel 70 and of the inner thermal shield 73 at various selected locations. Additional thermocouplesmeasure the temperature at various locations on *the upper surface of the upper and lower plates S3 and 84 ofthe bottom support 75. The temperatures of these structural components are valuable in calculating thermal stresses. These thermocouple leads also pass through the core support flange 62. There are eight penetrations in the flange for the 71 thermocouple leads that pass therethrough. A

The shroud instrumentation is designed so that all of the sensing elements can be removed and replaced With- All thermocouples Whose leads are removed through the closure head are individually replaceable. Individual guidetubes 250 (see FIGS. 21-24) are provided to guide each of the thermocouple or differential pressure tubes to their position overthe fuel assemblies and into the wells in the instrumented seed assembly. Although the guide tubes provide continuous runs, they are made up of three sections. One section runs from the entrance ou the mechanism housing to the top of upper shroud section 245; then another section is contained in the upper and middle shroud sections 245 and 246, and the third section is clamped in lthe scabbards 244 in the lower shroud section 247 and extends over the fuel assembly location. A fourth section of guide tubes is added in the top extension brackets 123 (see FIG. 30) and guides the thermocouples to their correct wells 154 in the instrumented seed fuel plate 153.

The exit Water temperature is taken at each fuel assembly. Thermocouples 304 are supported over each seed and blanket assembly by guide tubes 250 mounted in the shrouds 242 and in the module frames 243. These are shown in FIGS. 24 and 25 only, although they are mounted in every shroud 242 and on every module frame 243er, 24312, and 243C.

Guide tubes 250 for the therrnocouples 155 used in the instrumented seed assemblies are arranged the same as those described for the exit water thermocouples and no further description is believed necessary.

The core differential pressure is taken at three locations. v A static pressure probe is mounted in a guide tube 306 (see FIG. 27) over the central blanket fuel assembly 58 and over two other fuel assemblies in the core 53 and is used in conjunction with the upstream pressure tap 96 of the owmeter 93 in the same fuel assembly to measure the core differential pressure. The guide tube 306 is mounted in the same manner as are the guide tubes 250 for the removable thermocouples. The static pressure guide tube 306 is mounted so that its end extends into the flow stream at the top exension bracket 123 and holes in the side of ythe guide tube are located so as to obtain a true static pressure. The guide tube then follows the same path as the thermocouple Y guide tubes up to the mechanism housing.

Listed below are the design parameters for the reactor core described.

Table l SUMMARY OF MECHANICAL DESIGN PARAMETERS Active core height (it.) 7. 5 Mean diameter (ft.) 7. 0 U-235 loading in seed (kg.) 140 {+(l)5 U02 loading in blanket (tous) 22 Number of seed assemblies--. 20 Number of blanket assemblie 77 T able II OPERATING CHARACTERISTICS OF FOUR-LOOP OPERATION Y [Flow distribution chosen for no re-orifcing] General:

Thermal output, MW 350 Mw (th),

. f Mw (e). Total coolant flow, 1b./hr. l0 29.0. Leakage flow, lb./hr. 10 (5.8% of total) 1.68. Pressure vessel pressure drop, p.s.1 53.9.

Pressure vessel inlet and exit. 12.0 Friction 30. Extension brackets 10, 3 Flow meter 1.6. Inlet coolant temperature, F. 506. Average coolant temperature, 523. Outlet coolant temperature, F 540.

100 5, 000 8,000 10, 000 hrs. hrs. hrs. hrs.

Seed:

Thermal output, MW 158 131. 5 124 116 Coolant how, lb./hr. 1 10. 9 10.9 10. 9 10.9 Velocity, ft./sec 21. 7 21. 7 21. 7 21.7 Heat transfer coccient, B.t.u./

hr.ft.2 F.X103 10.3 10.3 10.3 10.3 Heat iiux, avg., B.t.u./hr.ft.2

103 80 66.4 62. 6 58. 5 Heat ux, max., B.t.u./hr.ft.2

10-3 364 336 339 296 Maximum surface metal temperature 636 626 626 612 Regions of Orificing Region 1 Region 2 Region 3 Region 4 Bllaret Flow, lb./hr. 10=

Assemblies 9 28 24 16 Coolant flow/assembly,

1b./hr. 104 15.1 24. 9 23. 6 15.1 Total flow/region, 1b.]

hr. l0 1.4 7.0 5.6 2.4 Velocity, ft./sec 7.2 11.8 11.2 7.2 Heat transfer coemcient, y

B.t.u./hr.ft.2 F.X 10-3 4.1 6.1 5.9 4.1

100 5,000 8,000 10,000 hrs. hrs. hrs. hrs.

Blanket:

Thermal output, Mw 174. 5 201 209. 5 216. 5 Heat Eux, avg., LB.t.u./hr.it.2

103 35. 6 41. 0 42. 7 44. 2 Region 1:

Heat ux, max., B.t.u./hr.

ft.2X103 103 116 130 116 Max. surface metal temp 612 624 636 625 Region 2:

Heat ux, max., B.t.u./hr.-

ft.2 103 142 168 200 208 Max. surface metal temp 595 613 630 636 Region 3:

Heat flux, max., B.t.u./hr.

ft.2X10-3 132 162 198 184 Max. surface metal temp 596 614 636 628 Region 4:

Heat flux, max., B.t.u./hr.

ft.2 103 103 116 130 116 Max. surface metal temp.--" 612 624 636 625 

1. A NUCLEAR REACTOR COMPRISING A CORE CONTAINING A PLURALITY OF VERTICALLY DISPOSED SEED AND BLANKET FUEL ASSEMBLIES, SAID SEED ASSEMBLIES CONTAINING ENRICHED URANIUM AND SAID BLANKET ASSEMBLIES CONTAINING NATURAL URANIUM, EACH OF SAID SEED ASSEMBLIES INCLUDING A CONTROL ROD CHANNEL EXTENDING AXIALLY OF THE ASSEMBLY, CONTROL RODS HAVING OPERATING SHAFTS EXTENDING THEREABOVE CAPABLE OF AXIAL MOVEMENT IN THE CONTROL ROL CHANNELS, TUBULAR SHROUDS ENCLOSING SAID CONTROL ROD OPERATING SHAFTS, AND MODULE FRAMES ATTACHED TO EACH SHROUD ADAPTED TO BEAR ON A SEED ASSEMBLY AND ADJACENT BLANKET ASSEMBLIES TO RESTRAIN THEM FROM VERTICAL MOVEMENT. 